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Reconstructing Pliocene West Pacific Warm Pool Hydroclimate Using In Situ Microanalyses on Fossil Planktic Foraminifer Shells
Author(s) -
Wycech J.,
Kelly D. C.,
Fournelle J.,
Kitajima K.,
Kozdon R.,
Orland I. J.
Publication year - 2020
Publication title -
paleoceanography and paleoclimatology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.927
H-Index - 127
eISSN - 2572-4525
pISSN - 2572-4517
DOI - 10.1029/2019pa003772
Subject(s) - calcite , diagenesis , geology , seawater , paleontology , carbonate , oceanography , sea surface temperature , isotopes of oxygen , pleistocene , mineralogy , geochemistry , chemistry , organic chemistry
Abstract A controversial aspect of Pliocene (5.3–2.6 Ma) climate is whether El Niño‐like (El Padre) conditions, characterized by a reduced trans‐equatorial sea‐surface temperature (SST) gradient, prevailed across the Pacific. Evidence for El Padre is chiefly based on reconstructions of sea‐surface conditions derived from the oxygen isotope (δ 18 O) and Mg/Ca compositions of shells belonging to the planktic foraminifer Trilobatus sacculifer . However, fossil shells of this species are a mixture of multiple carbonate phases—pre‐gametogenic, gametogenic (reproductive), and diagenetic calcites—that formed under different physiological and/or environmental conditions and are averaged in conventional whole‐shell analyses. Through in situ measurements of micrometer‐scale domains within Pliocene‐aged shells of T. sacculifer from Ocean Drilling Program Site 806 in the western equatorial Pacific, we show that the δ 18 O of gametogenic calcite is 0.6–0.8‰ higher than pre‐gametogenic calcite, while the Mg/Ca ratios of these two phases are the same. Both the whole‐shell and pre‐gametogenic Mg/Ca records indicate that average early Pliocene SSTs were ~1°C warmer than modern, with present‐day SSTs being established during the latest Pliocene and early Pleistocene (~3.0–2.0 Ma). The measurement of multiple calcite phases by whole‐shell δ 18 O analyses masks a late Pliocene to earliest Pleistocene (3.6–2.2 Ma) decrease in seawater δ 18 O (δ 18 O sw ) values reconstructed from in situ pre‐gametogenic δ 18 O and Mg/Ca measurements. Our novel δ 18 O sw record indicates that sea‐surface salinities in the west Pacific warm pool were higher than modern prior to ~3.5 Ma, which is consistent with more arid conditions under an El Padre state.